Doherty amplifier

ABSTRACT

A multistage linear power amplifier receiving an input signal. The multistage linear power amplifier comprises a plurality of Class-AB amplifiers connected in a cascade configuration. The plurality of Class-AB amplifiers amplifies the input signal to generate an amplified input signal. At least one of the plurality of Class-AB amplifiers is biased such that the multistage linear power amplifier emulates a Class-C amplifier.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from the U.S. provisional applicationNo. 61/576,243, titled “Composite Class C Doherty Peaking Amplifier”,filed on Dec. 15, 2011, the disclosure of which is hereby incorporatedby reference.

TECHNICAL FIELD

The presently disclosed embodiments are related, in general, to a poweramplifier. More particularly, the presently disclosed embodiments arerelated to a Doherty power amplifier.

BACKGROUND OF THE INVENTION

Mobile communication systems require broad bandwidth and high linearity.Communications signals, used in such mobile communication systems,exhibit a high peak-to-average power ratio. Typical RF power amplifiers,that are used to amplify the communication signals, are operated at alarge back-off power to satisfy the high peak-to-average power ratio andhigh linearity. Back-off corresponds to the difference between theincrease in power of the output signal with the increase in power of theinput signal. Typical RF amplifiers are described in conjunction withFIG. 1, FIG. 2, and FIG. 3.

FIG. 1 is a schematic diagram of a conventional multistage Class-ABpower amplifier 100. Multistage Class-AB power amplifier 100 includes aninput port 102, a multistage amplifier 104, and an output port 106.Multistage amplifier 104 includes an input amplifier 108, a driveramplifier 110, and an output amplifier 112. In an embodiment, inputamplifier 108 is Class-A biased, driver amplifier 110 and outputamplifier 112 are Class-AB biased.

Input port 102 is connected to input amplifier 108. Input amplifier 108,driver amplifier 110 and output amplifier 112 are connected in a cascadeconfiguration. Output amplifier 112 is connected to output port 106.

Multistage power amplifier 100 offers limited output power and limitedpower added efficiency. In order to improve the output power, twomultistage power amplifiers 100 are connected in parallel to form abalanced amplifier configuration as described in conjunction with FIG.2.

FIG. 2 is a schematic diagram of a conventional balanced amplifier 200.Balanced amplifier 200 includes a first power splitter 202, a firstmultistage amplifier 204, a second multistage amplifier 206, and asecond power splitter 208. First multistage amplifier 204 includes afirst input amplifier 210 a, a first driver amplifier 210 a and a firstoutput amplifier 214 a. Second multistage amplifier 208 includes asecond input amplifier 210 b, a second driver amplifier 210 b and asecond output amplifier 214 b.

The amplifier stages in first multistage amplifier 204 and secondmultistage amplifier 206 are biased in a similar manner as the amplifierstages in multistage amplifier 104 (refer to FIG. 1).

First power splitter 206 is connected to first input amplifier 210 a andsecond input amplifier 210 b. In first multistage amplifier 204, firstinput amplifier 210 a, first driver amplifier 210 a and first outputamplifier 214 a are connected in cascade configuration. Similarly, insecond multistage amplifier 206, second input amplifier 210 b, seconddriver amplifier 210 b, and second output amplifier 214 b are connectedin cascade configuration. First output amplifier 214 a and second outputamplifier 214 b are connected to second power splitter 208.

An input signal, applied at first signal splitter 202, is split in afirst signal and a second signal. The first signal and second signal are90 degrees out of phase. The first and second signals are then amplifiedby first multistage amplifier 204 and second multistage amplifier 206respectively. The amplified first and second signals are combined bysecond signal splitter 208 to generate an output signal.

As the signal is split (i.e., into the first signal and second signal)and amplified individually, the output power is improved as compared tomultistage Class-AB power amplifier 100. In an embodiment, balancedamplifier 200 produces approximately twice the output power as producedby multistage Class-AB power amplifier 100. In order to achieve furtherimprovement in the usable output power and to also improve the poweradded efficiency, balanced amplifier 200 is modified to form a Dohertyamplifier.

FIG. 3 is a schematic diagram of a conventional Doherty amplifier 300.Doherty amplifier 300 includes a main amplifier 302, a peaking amplifier304, and a Doherty combiner 306. Main amplifier 302 is Class B or ClassAB biased. Peaking amplifier 304 is Class C biased. Further, it is knownthat Doherty amplifier 300 may also include more than one peakingamplifiers.

Doherty amplifier 300 achieves high efficiency at back-off through mainamplifier 302 which operates into the high power added efficiencysaturation region. Further, due to class-C biasing in peaking amplifier304, peaking amplifier 304 supplies the signal peaks so that overalllinearity can be restored. Additionally, the Doherty amplifier 300achieves load modulation by using the principle of “load pulling” usingtwo devices (i.e., main amplifier and peaking amplifier).

Main amplifier 302 operates when main amplifier 302 receives a low powerinput signal. As the power of the input signal increases, the Class-Camplifier (i.e., peaking amplifier 304) turns ON abruptly. Such abruptturning ON of the Class-C amplifier leads to strong AM-AM distortion andAM-PM distortion. AM-AM distortion leads to undesired amplitudedeviations while amplifying the peaks of the communication signal.Similarly, AM-PM leads to undesired phase deviations while amplifyingthe peaks of the communication signal. As most of the analogcommunication signals carry digital symbols, AM-AM distortion and AM-PMdistortion may impede the ability to recognize the digital symbolsleading to a distortion known as Error Vector Magnitude (EVM).

A person having ordinary skill in the art will understand that AM-AMdistortions and AM-PM distortions in the output signal can be introduceddue to various other factors such as the non-linear characteristics ofamplifiers in the Doherty amplifier, and sudden gain compression andexpansion in the amplifiers.

A person having ordinary skill in the art will also understand that thelinearity degrades from Class-A to B and then to C while, in general,the current consumption decreases and power added efficiency (PAE)increases. Thus Class-A amplifier exhibits better linearity but at thecost of worse power added efficiency. Similarly, Class-C amplifierexhibits worse linearity but with the advantage of much improved poweradded efficiency. Such non-linear characteristics of Class-C amplifierintroduce inter-modulation distortions among one or more input signalsapplied to the Doherty amplifier.

An addition to the distortions, most of the Doherty amplifiers are bulkyas well as costly. Thus, such Doherty amplifiers may be unsuitable forvarious applications such as, but not limited to, active antennasystems, Femto cells, and mobile devices.

Thus, there is a need for an amplifier configuration that exhibits highpower added efficiency and gain.

SUMMARY OF THE INVENTION

According to embodiments illustrated herein, there is provided amultistage linear power amplifier receiving an input signal. Themultistage linear power amplifier includes a plurality of Class-ABamplifiers connected in a cascade configuration. The plurality ofClass-AB amplifiers amplify the input signal to generate an amplifiedinput signal. At least one of the plurality of Class-AB amplifiers isbiased such that the multistage linear power amplifier emulates aClass-C amplifier.

According to embodiments illustrated herein, there is provided a Dohertyamplifier. The Doherty amplifier includes a first multistage amplifierreceiving an input signal. The first multistage amplifier furtherincludes a first plurality of Class-AB amplifiers, connected in acascade configuration. The first plurality of Class-AB amplifiersamplifies the input signal. The Doherty amplifier further includes asecond multistage amplifier receiving a phase-shifted input signal. Thesecond multistage amplifier includes a second plurality of Class-ABamplifiers, connected in the cascade configuration. The second pluralityof Class-AB amplifiers amplifies the phase-shifted input signal. Atleast one of the second plurality of Class-AB amplifiers is biased suchthat the second multistage amplifier emulates a Class-C amplifier.

Since the main amplifier is a linear multi-stage AB amplifier, the mainamplifier exhibits good linear operation in the low power region.Further, since the peaking amplifier is a linear multi-stage ABamplifier, the peaking amplifier exhibits the lessened distortionbehavior of AB Class as opposed to the more severe and abrupt turn-onand distortion of Class-C. Thus, overall distortion is reduced incomparison to the prior art with Class-C peaking amplifier.

BRIEF DESCRIPTION OF DRAWINGS

The following detailed description of the embodiments of the presentinvention will be better understood when read in conjunction with theappended drawings. The present invention is illustrated by way ofexample, and not limited by the accompanying figures, in which likereferences indicate similar elements.

FIG. 1 is schematic diagram of a conventional multistage Class-AB poweramplifier;

FIG. 2 is a schematic diagram of a conventional balanced amplifier;

FIG. 3 is a schematic diagram of a conventional Doherty amplifier;

FIG. 4 is a schematic diagram depicting a multistage linear poweramplifier in accordance with an embodiment of the invention;

FIG. 5 is a schematic diagram depicting a Doherty amplifier inaccordance with an embodiment of the invention;

FIG. 6 is a graph illustrating variation in gain of a main amplifierwith respect to the output power in accordance with an embodiment of theinvention; and

FIG. 7 is a graph illustrating variation in gain of a peaking amplifierwith respect to the output power in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention can be best understood with reference to the detailedfigures and description set forth herein. Various embodiments arediscussed below with reference to the figures. However, those skilled inthe art will readily appreciate that the detailed description givenherein with respect to these figures is simply for explanatory purposes.The disclosed systems or circuits extend beyond the describedembodiments. For example, those skilled in the art will appreciate thatin light of the teachings presented, multiple alternate and suitableapproaches may be realized, to implement the functionality of any detaildescribed herein, beyond the particular implementation choices in thefollowing embodiments described and shown.

Definitions

The following terms shall have, for the purposes of this application,the respective meanings set forth below.

“Heavy Class-AB biasing” refers to 50% or more of Class-A (close toClass-A) biasing.

“Low Class-AB biasing” refers to 20% or below of Class-A (close toClass-B) biasing. Further, Low Class-AB biasing is divided into “VeryDeep Class-AB biasing” (below 4% of Class-A biasing) and “Deep Class-ABbiasing” (between 4% and 10% of Class-A biasing).

“Mid Range Class-AB biasing” refers to a range between “Low Class-ABbiasing” and “Heavy Class-AB biasing” (i.e., above 20% and below 50% ofClass-A biasing).

A person having ordinary skill in the art will understand that thereexists a continuum of an infinite number of possible bias points betweenClass-A and Class-B. For the purposes of explanation and readability,descriptive terms have been defined to distinguish different regions ofClass-AB biasing. A person having ordinary skill in the art will readilyrecognize that these are only descriptive terms and will furtherunderstand that the appropriate biasing point in the Class-AB continuumis dependent on a wide variety of factors and design/performancetrade-offs including, but not limited, to the particular signalcharacteristics, product application, and the particular semiconductortechnology/device technology.

FIG. 4 is a schematic diagram depicting a multistage linear poweramplifier 400 in accordance with an embodiment of the invention.Multistage linear power amplifier 400 includes an input port 402, amultistage amplifier 404, and an output port 412. Multistage amplifier404 further includes a first Class-AB amplifier 406, a second Class-ABamplifier 408, and a third Class-AB amplifier 410. In an embodiment,input port 402 is connected to first Class-AB amplifier 406 ofmultistage amplifier 404. First Class-AB amplifier 406, second Class-ABamplifier 408, and third Class-AB amplifier 410 are connected in acascade configuration to form multistage amplifier 404. Third Class-ABamplifier 410 is connected to output port 412.

In an embodiment, first Class-AB amplifier 406, second Class-ABamplifier 408, and third Class-AB amplifier 410 are configured as aninput stage, a driver stage, and an output stage of multistage amplifier404, respectively. A person having ordinary skill in the art willunderstand that scope of the invention is not limited to having threestages in multistage amplifier 404. In an embodiment, numbers ofamplifier stages may be determined based on gain requirement ofmultistage amplifier 404.

In an embodiment, first Class-AB amplifier 406 (input or pre-driverstage) is low Class-AB biased. Second Class-AB amplifier 408 (driverstage) is deep Class-AB biased. Third Class-AB amplifier 410 (or outputstage) is very deep Class-AB biased. A person having ordinary skill inthe art will understand that the biasing of first Class-AB amplifier406, second Class-AB amplifier 408, and third Class-AB amplifier 410 mayvary based upon many factors such as the application in which multistageamplifier 404 is to be used and the particular semiconductor technology.

U.S. Pat. No. 6,515,546, and assigned to the same assignee (AnadigicsInc), which is herein incorporated by reference in its entirety,discloses an exemplary biasing network that can be utilized for biasingfirst Class-AB amplifier 406, second Class-AB amplifier 408, and thirdClass-AB amplifier 410. However, the scope of the invention should notbe limited to the bias network of U.S. Pat. No. 6,515,546.

The operation of the multistage linear power amplifier 400 is describedlater in conjunction with FIG. 5.

FIG. 5 is a schematic diagram depicting a Doherty power amplifier 500 inaccordance with an embodiment of the invention. Doherty power amplifier500 is explained in conjunction with FIG. 4. Doherty power amplifier 500includes an input port 502, a signal splitter 504, a Doherty amplifier506, a Doherty combiner 516, and an output port 518. Doherty amplifier506 includes multistage amplifier 404 and a main amplifier 508.Multistage amplifier 404 includes first Class-AB amplifier 406, secondClass-AB amplifier 408, and third Class-AB amplifier 410. Main amplifier508 includes a fourth Class-AB amplifier 510, a fifth Class-AB amplifier512, and a sixth Class-AB amplifier 514. Hereinafter, multistageamplifier 404 has been referred as peaking amplifier 404.

For the purpose of the ongoing description, three amplifier stages havebeen considered for main amplifier 508. However, it will be apparent toa person having ordinary skill in the art that the number of amplifierstages in main amplifier 508 may vary based on the gain requirement ofmain amplifier 508. For example, if a plurality of amplifiers stages inmain amplifier 508 includes two amplifier stages i.e., a fourth Class-ABamplifier 510 and a fifth Class-AB amplifier 512, then fourth Class-ABamplifier 510 is configured as the driver stage of main amplifier 508and fifth Class-AB amplifier 512 is configured as the output stage ofmain amplifier 508.

Input port 502 is connected to signal splitter 504. Signal splitter 504is connected to fourth Class-AB amplifier 510 and first Class-ABamplifier 406. First Class-AB amplifier 406, second Class-AB amplifier408, and third Class-AB amplifier 410 are connected in a cascadeconfiguration to form peaking amplifier 404. Similarly, fourth Class-ABamplifier 510, fifth Class-AB amplifier 512, and sixth Class-ABamplifier 514 are connected in the cascade configuration to form mainamplifier 508. Sixth Class-AB amplifier 514 and third Class-AB amplifier410 are connected to Doherty combiner 516. Doherty combiner 516 isfurther connected to output port 518.

In an embodiment, fourth Class-AB amplifier 510 (input or pre-driverstage of main amplifier 508) is heavy Class-AB biased. Fifth Class-ABamplifier 512 (driver stage of main amplifier 508) is heavy Class-ABbiased. Sixth Class-AB amplifier 514 (output stage of main amplifier508) is low Class-AB biased. In an embodiment, peaking amplifier 404 isconstructed as an identical fabricated copy of the main amplifier 508but with differing biasing. First Class-AB amplifier 406, secondClass-AB amplifier 408, and third Class-AB amplifier 410 are biased asdescribed above in conjunction with FIG. 4.

Input port 502 receives the input signal from an external source (notshown). The input signal is transmitted to signal splitter 504. In anembodiment, signal splitter 504 is a 90 degree hybrid power splitter.Signal splitter 504 splits the input signal into a first signal and asecond signal. In an embodiment, the first signal and the second signalhave equal power (e.g., 3 dB). Further, the first signal and the secondsignal have a 90 degree phase difference. The first signal and thesecond signal are transmitted to fourth Class-AB amplifier 510 and firstClass-AB amplifier 406.

For the purpose of ongoing description, it is assumed that Doherty poweramplifier 500 achieves peak efficiency at 6 db back-off (i.e., thedifference between the increase in power of the output signal with theincrease in power of the input signal). However, the scope of theinvention is not to be limited to achieving peak efficiency at 6 dbback-off. A person having ordinary skill in the art will understand thatpeak efficiency of Doherty power amplifier 500 can be achieved at adifferent back-off value other than a 6 db back-off.

For instance, if the input signal is a low power signal (below 6 dbback-off), the first signal and the second signal generated by signalsplitter 504 will also be low power signals. As the first signal is alow power signal, main amplifier 508 operates in an ON state due to thebiasing of fourth Class-AB amplifier 510, fifth Class-AB amplifier 512,and sixth Class-AB amplifier 514. Further, main amplifier 508 exhibitsconstant gain as a function of power. FIG. 6 illustrates a graph 600showing the variation of gain of main amplifier 508 with increasingpower of the input signal. From the curve 602, it is observed that thegain of main amplifier 508 is constant with increasing power of theinput signal. Therefore, for low power signals, main amplifier 508outputs a linear amplified replica of the first signal received fromsignal splitter 504.

On the other hand, as the second signal is a low power signal and firstClass-AB amplifier 406 is low Class-AB biased, the gain of firstClass-AB amplifier 406 for a low power signal is very low in comparisonto the gain of main amplifier 508. FIG. 7 illustrates a graph 700showing the variation of gain of peaking amplifier 404 with increasingpower of the amplified second signal. From the curve 702, it is observedthat peaking amplifier 404 exhibits significantly reduced gain underback off due to the described biasing. Thus, the output of peakingamplifier 404 is a much smaller amplified replica of the second signalin comparison to output of main amplifier 508. Further, the currentconsumption of peaking amplifier 404 is small (due to low biascurrents). Therefore, peaking amplifier 404 emulates the behavior of aClass-C amplifier in a Doherty amplifier configuration (i.e., providingnegligible output when a low power signal is applied).

Additionally, for low power input signals, as output of peakingamplifier 404 is negligible, peaking amplifier 404 provides negligibleload modulation emulating the effect of a Class-C peaking amplifier thatoffers an open circuit to Doherty combiner 516. Therefore, the loadpresented to the main amplifier 508 approaches twice that of optimumoutput load impedance. In an embodiment, the optimum load impedance formain amplifier 508 and peaking amplifier 404 is 50 ohms.

As the power of the input signal increases (region between 0 db and 6 dbback-off), peaking amplifier 404 exhibits smooth gain expansion (due tothe self biasing of first Class-AB amplifier 406, second Class-ABamplifier 408 and third Class-AB amplifier 410). From curve 702, it canbe observed that the gain of peaking amplifier 404 increases smoothlywith increase in the power of the input signal. The smooth increase inthe gain of peaking amplifier 404, leads to smoother AM-AM and AM-PMdistortion characteristics with the absence of sudden changes ordiscontinuities.

Additionally, as the power of the input signal increases the efficiencyof main amplifier 508 reaches a peak and the gain of peaking amplifier404 starts increasing. Due to the increase in the gain of peakingamplifier (in the region between 0 dB and 6 dB back-off), the parallelconnection of the main and peaking amplifier 404 across the load throughDoherty combiner 516 results in load modulation.

As power of the input signal increases, the power of the first signaland the second signal (generated by signal splitter 504) also increases.As the power of the first signal and second signal increases, the gainand output power of peaking amplifier 404 both rises until both thepeaking amplifier 404 and the overall Doherty amplifier 506 reachessaturation and peak power added efficiency at the zero back-offcondition. As main amplifier 508 and peaking amplifier 404 are driveninto the saturation region, the gain of main amplifier 508 and peakingamplifier 404 is reduced. From curves 602 and 702, it is seen that thegain of main amplifier 508 and peaking amplifier 404 shows a dip after30 db power of the output signal.

Post the amplification, signal combiner 508 combines the amplifiedsecond signal and the amplified first signal to generate an outputsignal. The out signal is outputted from output port 518.

A person having ordinary skill in the art will understand that singlepeaking amplifier 404 in Doherty amplifier 500 has been shown forillustrative purposes. In an embodiment, Doherty amplifier 500 mayinclude more than one peaking amplifiers connected in parallel with mainamplifier 508. Various such Doherty amplifier configurations having morethan one peaking amplifiers have been disclosed in the U.S. patentapplication Ser. No. 13/548,774, filed Jul. 10, 2012 entitled“INTEGRATED OUTPUT COMBINER FOR AMPLIFIER SYSTEM”, assigned to the sameassignee (ANADIGICS, INC.), which is herein incorporated by reference inits entirety. It should be apparent to a person having ordinary skill inthe art that the biasing as disclosed in various embodiments of thepresent invention can be applied to the one or more peaking amplifiersto achieve further improvements.

The embodiments of the present invention provide several advantages. Asmain amplifier 508 and peaking amplifier 404 include a plurality ofClass-AB amplifiers, main amplifier 508 and peaking amplifier 404 can beconstructed as identical fabricated copies. Constructing identicallyfabricated copies of main amplifier and peaking amplifier prevents theproblems caused by manufacturing/process variations due to the inherentmatching of MMIC (Monolithic microwave integrated circuit) diesfabricated side by side on a semiconductor wafer. Identically fabricatedcopies leads to closely matched gain and phase in the Doherty poweramplifier containing two (or more) multistage amplifier paths withoutthe use of costly and space consuming circuitry for calibration orcompensation of the gain and phase balance. The conversion of anexisting linear multistage amplifier into a Doherty-like amplifier hasstrong positive business and manufacturing implications such as time tomarket, stability over temperature range and relative immunity toprocess/manufacturing variation. In addition, this technique enables anexisting linear amplifier to be converted to a Doherty power amplifierwith higher power and higher efficiency. Further, peaking amplifier 404exhibits a smoother gain expansion in comparison to sudden turn-on inClass-C amplifier (as used in the prior art). Therefore, distortions,such as the AM-AM distortion and the AM-PM distortion, due to suddenturn-on are reduced.

While various embodiments of the present invention have been illustratedand described, it will be clear that various multistage amplifiers (themain amplifier and one or more peaking amplifiers) of the Dohertyamplifier can be fabricated as a single integrated circuit, or asdiscrete circuit components connected together. Further, various otherpossible combinations of electronic components of the multistageamplifiers may also be used without departing from the scope of theinvention.

While various embodiments of the present invention have been illustratedand described, it will be clear that the invention is not limited tothese embodiments only. Numerous modifications, changes, variations,substitutions and equivalents will be apparent to those skilled in theart, without departing from the basic scope and spirit of the invention,as described in the claims that follow.

1-19. (canceled)
 20. A multistage power amplifier comprising: aplurality of amplifiers biased within a bias range associated with anamplifier of a first class, at least one of the plurality of amplifiersbiased within the bias range is biased within a first portion of thebias range and at least one other of the plurality of amplifiers biasedwithin the bias range is biased within a second portion of the biasrange that differs from the first portion of the bias range such thatthe multistage power amplifier mimics a behavior of an amplifier of asecond class while each of the plurality of power amplifiers is biasedwithin the bias range associated with the amplifier of the first class.21. The multistage amplifier of claim 20 wherein the plurality ofamplifiers are connected in series.
 22. The multistage amplifier ofclaim 20 wherein amplifiers of the first class exhibit a differentlinearity than amplifiers of the second class.
 23. The multistageamplifier of claim 20 wherein amplifiers of the second class exhibitdifferent power added efficiency than amplifiers of the first class. 24.The multistage amplifier of claim 20 wherein the amplifier of the firstclass is a Class-AB amplifier and the amplifier of the second class is aClass-C amplifier.
 25. The multistage amplifier of claim 20 whereinamplifiers of the first class exhibit greater linearity and lower poweradded efficiency than amplifiers of the second class.
 26. A Dohertypower amplifier comprising: a first multistage power amplifier includinga first plurality of amplifiers of a first class, the first multistagepower amplifier configured to amplify a first signal; and a secondmultistage power amplifier configured to amplify a second signal that isa phase shifted version of the first signal, the second multistage poweramplifier including a second plurality of amplifiers biased within abias range associated with an amplifier of the first class, at least oneof the second plurality of amplifiers biased within a first portion ofthe bias range and at least one other of the second plurality ofamplifiers biased within a second portion of the bias range that differsfrom the first portion of the bias range such that the second multistagepower amplifier mimics a behavior of an amplifier of a second classwhile each of the second plurality of power amplifiers is biased withinthe bias range associated with the first class of amplifier.
 27. TheDoherty power amplifier of claim 26 wherein the first signal and thesecond signal are of equal power and have a phase difference of 90degrees.
 28. The Doherty power amplifier of claim 26 further comprisinga splitter configured to split an input signal into the first signal andthe second signal.
 29. The Doherty power amplifier of claim 26 furthercomprising a signal combiner configured to combine an output of thefirst multistage power amplifier and an output of the second multistagepower amplifier to obtain an output signal of the Doherty poweramplifier.
 30. The Doherty power amplifier of claim 26 wherein the firstmultistage power amplifier is a main amplifier and the second multistagepower amplifier is a peaking amplifier.
 31. The Doherty power amplifierof claim 30 wherein an amplification of the peaking amplifier is lessthan an amplification of the main amplifier during back off operation.32. The Doherty power amplifier of claim 26 wherein the first multistagepower amplifier and the second multistage power amplifier include thesame number of power amplifier stages.
 33. The Doherty power amplifierof claim 26 wherein at least one amplifier of the first multistage poweramplifier is biased differently than at least one amplifier of thesecond multistage power amplifier.
 34. The Doherty power amplifier ofclaim 33 wherein a position within the first multistage power amplifierof the at least one amplifier of the first multistage power amplifiercorresponds to a position within the second multistage power amplifierof the at least one amplifier of the second multistage power amplifier.35. The Doherty power amplifier of claim 26 wherein at least one otherof the second plurality of amplifiers is biased within a third portionof the bias range that differs at least in part from the first portionand the second portion of the bias range.
 36. The Doherty poweramplifier of claim 26 wherein the first plurality of amplifiers areconnected in series and the second plurality of amplifiers are connectedin series.
 37. The Doherty power amplifier of claim 26 whereinamplifiers of the first class exhibit a different linearity thanamplifiers of the second class.
 38. The Doherty power amplifier of claim26 wherein amplifiers of the second class exhibit different power addedefficiency than amplifiers of the first class.
 39. The Doherty poweramplifier of claim 26 wherein the amplifier of the first class is aClass-AB amplifier and the amplifier of the second class is a Class-Camplifier.